Carbon nanofibers are a considerably different material in structure and size from carbon fibers being widely used at present. Carbon nanofibers have a similar size to multi-walled carbon nanotubes but a different structure from them. Carbon nanotubes are constructed of sp2 bonded carbon atomic layers of a hexagonal shape, the carbon atomic layers arranged in layers in cylindrical form parallel to the axial direction, while carbon nanofibers have a structure in which identical carbon atomic layers are stacked forming an angle with the axial direction.
Although carbon nanofibers do not have higher tensile strength and electrical conductivity than carbon nanotubes, carbon nanofibers have a structure in which edges of each carbon atomic layer are exposed outside, so they are suitable as nanomaterials requiring a high surface energy.
For several tens nanometer scale growth mechanism of carbon nanotubes and carbon nanofibers using a catalytic chemical vapor deposition (CVD) method, Baker has discovered that bulk diffusion of carbon atoms is a factor critical in determining a final length or a reaction rate.
There are many carbon nanofibers growth methods, and a carbon nanofibers growth method using a most general CVD method is summarized as follows:
(a) Above a metal catalyst where hydrocarbon gas such as ethylene or methane is deposited on the surface, carbon and hydrogen separate, and hydrogen in gaseous state escapes, with only carbon atoms left.
(b) The carbon atoms are accumulated within catalyst particles by diffusion, and when the catalyst particles exceed the limit of capabilities of accommodating carbon atoms, the carbon atoms are accumulated on the surface of the catalyst particles.
(c) When carbon atoms are continuously supplied to the surface of the catalyst particles, carbon nanofibers are grown.
A method of producing carbon nanofibers using a catalyst includes a method using a floating (not fixed) catalyst and a method using a catalyst supported on a substrate, and if a method using a catalyst supported on a substrate is used, carbon nanofibers may be grown vertically like trees in the forest. Generally, the thickness of carbon nanofibers depends on a catalyst particle size, and a catalyst is present at one end part of carbon nanofibers and continuously synthesizes carbon nanofibers during activation.
Generally, when iron is used as a catalyst, carbon nanotubes rather than carbon nanofibers are formed, and it is thought that it is because both bulk diffusion and surface diffusion are concurrently active. A metal that does not instigate bulk diffusion as strongly as iron but comes off second best is palladium. It is reported that if palladium is used, primarily carbon nanofibers rather than carbon nanotubes are synthesized, and if plasma is used together, synthesis is accomplished much more easily.
There have been many reports on vertically grown millimeter-scale carbon nanotubes with a very high aspect ratio, but vertically grown carbon nanofibers with a very aspect ratio have not been reported so far.